The present invention relates to high temperature vacuum heating of crucibles, for example, for vacuum evaporation.
Numerous constructions of sources of devices are already known which are suitable for effecting vacuum evaporation of a body heated to high temperature, said sources generally comprising a refractory crucible of suitable shape containing the body to be evaporated, which is normally a metallic product. The crucible temperature is raised by means of electrical resistance units applied to the outer surface of the crucible. In conventional solutions, the crucible is made from a ceramic material, preferably beryllium oxide, and the electrical resistance units comprise tantalum members fixed to the crucible and connected to a power supply, whereby a heat release takes place in these members by Joule effect. In particular, and as described in French Pat. No. 1,389,582, in the name of the Commissariat a l'Energie Atomique, the crucible advantageously has a conical shape and the heating elements associated with the crucible comprise a metal strip shaped to precisely follow the crucible profile so as to be in contact with the entire outer surface thereof, thus bringing about an adequate coupling and preventing a non-homogeneous distribution of the heat produced and mechanical stresses in the crucible thickness.
However, in constructions of this type, experience has shown that there is a disadvantage, particularly in the case where the temperature necessary for evaporating the body or metallic product contained in the crucible exceeds 1600.degree. C. Under such conditions, a reaction takes place between the tantalum of the resistance units and the beryllium oxide of the crucible, which considerably limits the service life of the latter, whereby the service life decreases as the temperature increases. Moreover, at such a temperature, particularly in the case where the crucible contains molten aluminium, the latter diffuses through the crucible wall and forms with the tantalum of the resistance units an alloy, whose melting point is lower, thus causing the destruction of the resistance units. In general, even under the best conditions, the service life of the crucible would not significantly exceed 5 minutes.